1. Field of the Invention
The present invention relates to a power supply circuit.
2. Description of the Related Art
As a power supply circuit for various electronic apparatuses, a power supply circuit that has a rectifier circuit (e.g., a diode bridge circuit) and a smoothing circuit (e.g., a condenser) and converts an alternating-current voltage into a direct-current voltage is known. However, such a power supply circuit is poor at power factor because, when an alternating voltage in the form of a sine wave is applied, a current flows through the power supply circuit only around the peak of the sine wave. Furthermore, where even with applying a sine wave alternating voltage, a current not proportional to a sine wave flows, harmonics that occur may affect adversely equipment around it.
Accordingly, a power supply circuit has been proposed wherein a switching element is connected in parallel with a diode and wherein by having the switching element switch at appropriate timings, the current flowing through the power supply circuit is made similar in shape to the sine wave of the alternating voltage, thereby improving power factor, suppressing harmonics, and adjusting the direct-current voltage. See, for example, Japanese Patent Application Laid-Open Publication No. 2001-286149.
FIG. 8 is a block diagram illustrating the configuration of a conventional power supply circuit. The conventional power supply circuit comprises four diodes D11, D12, D13, D14 forming a diode bridge circuit; a resistor R11 connected between the connection point of the anodes of the diodes D11, D12 and an N side electrode, the resistor R11 with which to detect the currents flowing through the diodes D11, D12; a switching element 114 connected in parallel with the diode D11 and the resistor R11; and a switching element 116 connected in parallel with the diode D12 and the resistor R11. Note that the switching elements 114 and 116 are, for example, bipolar transistors.
An alternating voltage generated by an alternating current power supply 110 is applied to the connection point of the diodes D11, D13 via a reactor 112. Moreover, an alternating voltage generated by the alternating current power supply 110 is directly applied to the connection point of the diodes D12, D14.
Hereinafter, the alternating voltage input line from the alternating current power supply 110 to the connection point of the diodes D11, D13 is referred to as an R line, and the alternating voltage input line from the alternating current power supply 110 to the connection point of the diodes D12, D14 is referred to as an S line.
An alternating voltage input from the alternating current power supply 110 is full-wave-rectified by the diode bridge circuit (D11 to D14), and then is smoothed by a condenser 122 connected between a positive side electrode (hereinafter, called a P side electrode) and a negative side electrode (hereinafter, called the N side electrode), which act as the output.
Note that the timings when the switching elements 114 and 116 switch are controlled by a control circuit (not shown) based on the current flowing through the resistor R11.
FIG. 9 is a diagram for explaining the current flows in the conventional power supply circuit for when the alternating voltage is positive. When the alternating voltage is positive, the switching element 114 switches on/off, and thereby current paths are switched.
<<When the Switching Element 114 is On>>
When the alternating voltage on the R line is positive relative to the S line and the switching element 114 is on, a current flows through a path indicated by the dot-dashed line in FIG. 9, that is, a path from the R line (the reactor 112) to the switching element 114 to the resistor R11 to the diode D12 to the S line. During this period of time, energy is stored in the reactor 112.
<<When the Switching Element 114 is Off>>
When the alternating voltage on the R line is positive relative to the S line and the switching element 114 is off, the reactor 112 induces a current to flow in the same direction as the current direction for when the switching element 114 is on. Thus, a current flows through a path indicated by the broken line in FIG. 9, that is, a path from the R line (the reactor 112) to the diode D13 to the condenser 122 to the resistor R11 to the diode D12 to the S line. During this period of time, the energy stored in the reactor 112 is output to the condenser 122. The condenser 122 is charged, and a direct-current voltage generated between the P side electrode and the N side electrode (hereinafter, called a direct-current output voltage) is raised in voltage level.
FIG. 10 is a diagram for explaining the current flows in the conventional power supply circuit for when the alternating voltage is negative. When the alternating voltage on the R line is negative relative to the S line, the switching element 116 switches on/off, and thereby current paths are switched.
<<When the Switching Element 116 is On>>
When the alternating voltage on the R line is negative relative to the S line and the switching element 116 is on, a current flows through a path indicated by the dot-dashed line in FIG. 10, that is, a path from the S line to the switching element 116 to the resistor R11 to the diode D11 to the R line (the reactor 112). During this period of time, energy is stored in the reactor 112.
<<When the Switching Element 116 is Off>>
When the alternating voltage on the R line is negative relative to the S line and the switching element 116 is off, the reactor 112 induces a current to flow in the same direction as the current direction for when the switching element 116 is on. Thus, a current flows through a path indicated by the broken line in FIG. 10, that is, a path from the S line to the diode D14 to the condenser 122 to the resistor R11 to the diode D11 to the R line (the reactor 112). During this period of time, the energy stored in the reactor 112 is output to the condenser 122. The condenser 122 is charged, and the direct-current output voltage is raised in voltage level.
The timings of switching operation of the switching element 114 or 116 have been controlled based on the current flowing through the resistor R11 connected in series to the diodes D11, D12. By repeating the switching operation, the direct-current output voltage is raised, and the current flowing through the power supply circuit is regulated to become similar in shape to the sine wave of the alternating voltage.
In the conventional power supply circuit, the diode D11 and the resistor R11 are connected in series, and in parallel with them, the switching element 114 is connected. Furthermore, the diode D12 and the resistor R11 are connected in series, and in parallel with them, the switching element 116 is connected.
Hence, regardless of whether the alternating voltage is positive or negative and whether the switching elements 114, 116 are on or off, a current always flows through the resistor R11, which flows through the diode D11 or the diode D12.
Thus, with such a conventional power supply circuit, there is the problem that since a current always flows through the resistor R11, which flows through the diode D11 or the diode D12, power loss exists thus reducing efficiency. Moreover, when the alternating voltage starts to be input, a rush current to charge the condenser 122 at the output flows. Thus, there is also the problem that power capacity need be enlarged.